docs: def get_document_unit(self):
          """Get the unit of the SVG surface.
  
          If the surface passed as an argument is not a SVG surface, the function
          sets the error status to ``STATUS_SURFACE_TYPE_MISMATCH`` and
          returns :ref:`SVG_UNIT_USER`.
  
          :return: The SVG unit of the SVG surface.
  
          *New in cairo 1.16.*
  
          *New in cairocffi 0.9.*
  
          """
          unit = cairo.cairo_svg_surface_get_document_unit(self._pointer)
          self._check_status()
          return unit